It's been remarkable to see how quickly the Democratic Party has coalesced around Hillary Clinton as its expected 2016 nominee, despite clear vulnerabilities she's telegraphed during her book tour. Clinton brings undeniable assets to the table—she'd be the first female president, the Clinton brand is still strong, her fundraising is unmatched—but her recent exposure on the book tour has demonstrated her political limitations as well.

I've outlined some of them in pastcolumns: She's not a particularly good campaigner; she's skilled at staying on message but tone-deaf to the way comments about her wealth could backfire among an economically anxious public. With the threat of terrorism rising and increased turbulence in Ukraine, Syria, and Iraq, Clinton could find that her record as secretary of State is a major vulnerability in an election where foreign policy is looming as a major issue. Most important, she tied herself to President Obama by accepting his offer to run State, assuming that his coattails would be awfully valuable down the road. Now, with Obama's approval ratings tanking, scandals abounding, and a new Quinnipiac poll showing a plurality of voters consider him the "worst president" since World War II, Clinton knows she needs to keep some distance from Obama while maintaining the excitement of his base. That's not a great place to be.

Her biggest asset is the fact that the entire Democratic Party infrastructure is behind her, seemingly resigned to her vulnerabilities but hopeful about her potential. Even progressives who are nervous about her Wall Street connections are merely hoping to nudge her leftward, and not aggressively challenge her with an actual candidate. With a lackluster Democratic bench, it's hard to find many alternatives even willing to throw their names out there. And let's be clear: Former Montana Gov. Brian Schweitzer, whose loose lips would sink a campaign before it launched, and Sen. Bernie Sanders of Vermont, throwing in his name as a protest candidate, don't qualify.

That doesn't mean there aren't credible candidates who, on paper, could mount a serious challenge. With anti-Washington sentiment running high, this is a promising opportunity for an outsider to run and surprise. True, they don't seem to want to run, whether from fear of the Clinton machine, a desire to avoid challenging someone who might make history, or simply an assumption that 2016 isn't a great year for Democrats.

But the candidates exist. Here are some prospects who would normally be touted for higher office but have acquiesced to Hillary Clinton in the run-up to the 2016 election.

1. Sen. Tim Kaine of Virginia

Kaine was one of the first Democratic officials to jump on the Obama bandwagon, and he has a resume that normally would be the envy of his fellow pols: swing-state governor; Democratic National Committee chairman; senator elected on Obama's coattails against a former GOP presidential prospect, George Allen. Kaine was on the very short list of potential Obama running mates. If this were the resume of a Republican candidate, it would vault him to the top of the list of 2016 front-runners.

But instead, Kaine took the unusual step in May of endorsing Clinton before she even announced her candidacy, perhaps angling for a Cabinet post over pursuing any possible national ambitions. Maybe being a white man in the Democratic Party is now a vulnerability in the Obama era, but Kaine certainly could score chits as an early Obama supporter who helped swing his state the president's way. And his Midwestern roots, authentic personality (in sharp contrast to Clinton), and executive experience would all be strong selling points to a national audience.

2. Massachusetts Gov. Deval Patrick

One of the obvious, yet underappreciated, factors in Obama's upset of Clinton was how powerful a role race played in the 2008 presidential primaries. Clinton had close ties to the African-American community from her days in the White House, but once it became clear that Obama was a serious challenger, he overwhelmingly carried the black vote in nearly every primary state where it mattered.

Why couldn't that dynamic repeat itself in 2016? Massachusetts Gov. Deval Patrick is leaving office, and he is a close ally of Obama's. (Obama even touted him as a prospective candidate.) Unlike the 2008 version of Obama, Patrick boasts executive experience as a two-term governor who had to deal with one of the biggest crises during the Obama presidency—the Boston Marathon bombings. Unlike Mitt Romney before launching his first presidential campaign, Patrick scored solid approval ratings in his last year in office (53 percent in a January 2014 MassINC poll).

Patrick recently said he worries about how Clinton is being viewed as the inevitable nominee, but he hasn't made any moves of his own to suggest he's running. But if he could put a credible team together, he'd be a much more threatening challenger than, say, Maryland Gov. Martin O'Malley.

3.Sen. Claire McCaskill of Missouri

In a normal year, a female media-savvy, red-state prosecutor who defied the odds to win a second term in the Senate would be at the top of many Democratic wish lists. But like Kaine, this early Obama supporter was one of the first elected officials to sign up with Clinton's nascent campaign, taking herself out of the conversation. Part of her motive was to ingratiate herself with Team Clinton, who placed McCaaskill on Hillary's "enemies list" after she said she didn't want her daughter near the former president in a Meet the Press interview (as an Obama surrogate).

Instead of sucking up to the Clintons, why not challenge Hillary? Representing a populist state, McCaskill would be well positioned to challenge Clinton on her wealth, ties to corporations, and perceived disconnect from the middle class. Plus, McCaskill's long-term prospects in the Senate aren't great, assuming she doesn't face Todd Akin again in 2018.

4. Former Sen. Russell Feingold of Wisconsin

Where have you gone, Russ Feingold? The former Wisconsin senator and campaign finance reform scold has virtually disappeared from the political arena. Like Clinton, he's now serving in the State Department—as the special envoy for the African Great Lakes region and the Democratic Republic of the Congo.

Like Elizabeth Warren, Feingold would be able to rally progressives around his campaign but he could potentially have more appeal to male voters, a demographic where the party has gotten crushed in the Obama era. Unlike Clinton (and Warren), Feingold took a lone stand for same-sex marriage in 2006, when most elected Democrats opposed such legislation. He's been a longtime critic of outside groups' campaign spending, which has been a rallying cry for liberal Democrats in the age of the super PAC.

Feingold has always marched to the beat of his own drum, and it would be hard to see him prevailing over the better-organized Clinton. But he could persuasively assert he was ahead of the curve on the issues animating today's Democratic Party, a powerful argument for the grassroots base. Indeed, he'd be in a situation similar to that of another reform-minded former Democratic senator, Bill Bradley, who challenged a sitting vice president and nearly won the New Hampshire primary.

5. Missouri Gov. Jay Nixon

Winning two terms in an increasingly Republican red state—he ran 9 points ahead of Obama in 2008 and 11 points ahead in 2012—Nixon is one of the most accomplished Democratic governors in the country. TheKansas City Star's Steve Kraske dubbed Nixon the "Teddy Roosevelt of Missouri—vigorous, a champion of the outdoors, constantly touring all corners of the state more than any chief executive in state history." He worked with Republicans to pass comprehensive jobs legislation, cut spending, and passed ahead-of-the-curve legislation incentivizing college graduates to specialize in high-demand health care fields. Nixon won high praise for his handling of the aftermath of the tornadoes that devastated Joplin. And he's won over some social conservatives by allowing restrictions on late-term abortions and reducing the age for residents to purchase a concealed-weapons permit. But he's also expanded Medicaid and focused on boosting spending for education.

In short, his positions on social issues would probably be untenable in today's Democratic Party, where moderates are becoming as extinct as their counterparts in the Republican Party. And Nixon has shown no interest in national office, knowing the near-insurmountable challenges he'd face in a primary.

In 1992, when Democrats nominated a centrist Southern governor as their presidential nominee, it was a move born out of weakness, with party leaders desperately seeking to moderate their image and initially holding little hope they could oust the sitting president. At the onset of the primary, the field was wide open, with the party's biggest-name contenders (Mario Cuomo, Al Gore) opting not to run. The situation could well be reversed in 2016: Democrats acting like they're in a stronger position than the reality, opting for a coronation instead of a contested primary, and ignoring the political logic of nominating an electable moderate outsider who can expand the party's coalition. In 1992's more ideologically diverse Democratic Party, Nixon would be at the top of many Democratic wish lists. But we're still stuck in Clintonworld.

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